The invention is in the field of power integrated circuit (PIC) devices, and relates more specifically to the prevention of latch-up in low-power circuit elements provided on the same integrated circuit as a power device.
In such PIC devices, a free-wheeling diode is typically used in conjunction with the power device in inductive load power switching and other applications. This free-wheeling diode is typically a separate, external diode connected to prevent forward current from flowing through the power device in such a manner as to inject minority carriers into the substrate. It would be desirable, however, to eliminate the external diode by having the body diode formed by adjacent regions of opposite conductivity types in the power device serve as the free-wheeling diode. This configuration not only eliminates an external part, but also reduces the pin count when used in a PIC device.
However, in PIC devices such as those built using a bulk CMOS process, when the body diode is turned on, minority carriers will be injected into the substrate. Since the low power circuits are fabricated in a semiconductor well which can form the collector of a parasitic transistor, minority carriers injected by the body diode will be collected by the well, resulting in latch-up and failure of the low-power circuits. Thus, in order to use the body diode in a PIC device as a free-wheeling diode, a technique is needed to prevent the interaction between the body diode and the low-power circuit elements fabricated in the semiconductor well. Although various prior-art techniques for preventing latch-up and/or absorbing minority carriers are known (see, for example, U.S. Pat. Nos. 4,952,998 and 4,862,233, Japanese Kokai 62-12151, and Epitaxial Layer Enhancement of n-Well Guard Rings for CMOS Circuits, IEEE Electron Device Letters, Vol. Edl-4, No. 12, pp. 438-440, December, 1983), none of these prior-art techniques are completely satisfactory for preventing latch-up when using the body diode of a power device as a free-wheeling diode in a power integrated circuit.